Most carbon blacks are currently produced by furnace processes in which a feed-stock hydrocarbon is thermally decomposed in the presence of hot gases produced by combustion of fuel. A carbon black-containing aerosol results in which the carbon black is suspended in the hot combustion gases and the other gaseous products which result from the thermal decomposition of the feedstock. The resultant aerosol is then prequenched or cooled to stop the carbon black forming reaction, often at least partially with water, prior to passage into a collection system for further quenching and recovery of the carbon black.
The interval between the introduction of the feedstock into the hot combustion gases and the prequenching is critical to the nature of the carbon black products and the nature of the carbon black products is thought to confer different properties upon the materials with which the product carbon black is compounded.
The interest in the nature and character of the carbon black product resulted in the development of numerous tests to characterize the carbon black. Some of these tests have developed as ASTM standards. These include such tests as: Dibutyl Phthalate Absorption Number of Compressed Sample (ASTM D 3493-76), hereinafter referred to as 24M4DBP; Dibutyl Phthalate Absorption Number (ASTM D 2414-76), hereinafter referred to as DBP; Surface Area (ASTM D 3037-76), hereinafter referred to as N.sub.2 SA; Toluene Discoloration (ASTM D 1618-75), hereinafter referred to as Photelometer; Iodine Adsorption Number (ASTM D 1510-76), hereinafter to as I.sub.2 No.; Tint Strength (ASTM D 3265-76), hereinafter referred to as Tint; and CTAB (ASTM D 3765-79), hereinafter referred to as CTAB.
Other tests have not developed into ASTM standards but are performed according to test procedures established by manufacturers and users of carbon black. These tests include such tests as "Cabot" Tint Factor, "Cabot" Iodine surface area, and the like, such as described, for example, in U.S. Pat. Nos.: 3,725,103; 3,799,788; 3,830,774; 3,864,305; 3,973,983; and 4,035,336.
Carbon black has been characterized by its optical properties, such as reflectance or absorbance of light, since before the furnace process became widely used; and in recent years a revival of interest in optical characterization has occurred. Particularly popular in recent years has been the tint strength test performed according to ASTM D 3265-76. In this test, the decrease in reflectance of a white pigment as a result of addition of carbon black is recorded. It can now be stated firmly that measuring the surface reflectance of a paste of carbon black in this manner provides a relative measure of average optical size, which is a function of the total morphology of the carbon black being measured.
Total morphology, however, embraces a number of factors. As is well known from morphology studies, carbon black entities consist of basic or ultimate particles known as nodules which are fused together into structural aggregates of varying size and configuration. Total morphology of the carbon black therefore embraces a number of factors: nodule size, shape, the degree of structuring (hereinafter referred to as "structure"), and breadth of distributions of all these factors. This latter distribution factor, which cannot be measured routinely, is lumped into a property referred to as "distribution" or "aggregate size distribution".
Tint strength, as measured by ASTM D 3265-76, increases with surface area, decreases with increasing structure, and increases as breadth of distribution narrows. So high tint strength does not necessarily mean high surface area (and therefore high "quality", as is commonly believed). It means high surface area only if such factors as structure and breadth of distribution are held constant. In recent years, however, it has been learned to vary structure over quite a range, and therefore if surface area and breadth of distribution are held constant while reducing structure, the resulting "high tint strength" carbon black could be poorer quality, at least so far as road wear is concerned.
It can thus be seen that the tint strength test by itself suffers shortcomings in that it measures and is sensitive to a combination of at least three basic factors of carbon black. A tint strength number, by itself, can thus be misleading unless accompanied by other information, specifically a measure of nonporous surface area and an accurate measure of structure. These other characteristics can be provided by ASTM tests: CTAB (ASTM D 3765-79); N.sub.2 SA (ASTM D 3037-76); and 24M4DBP (ASTM D 3493-76).
The concept of tint residual came about as a result of an effort to make tint strength according to ASTM D 3265-76 supply information about distribution. Since tint strength is basically determined by the variables of surface area, structure, and breadth of distribution, a regression analysis was performed on a number of carbon blacks in terms of surface area and structure alone. The neglected variable, breadth of distribution, then appeared as deviations from the regression line or "residuals" (hereinafter referred to as "tint residual"), i.e., EQU tint residual=Tint.sub.ASTM -Tint.sub. MODEL
where
Tint.sub.MODEL =56+1.067S.sub.c -0.00275S.sub.c.sup.2 -0.2596A-0.201(S.sub.n -S.sub.c); PA1 S.sub.c =Specific Surface Area (CTAB) (as measured by ASTM D 3765-79), m.sup.2 /g; PA1 S.sub.N =Specific Surface Area (N.sub.2 SA) (as measured by ASTM D 3037-76), m.sup.2 /g; PA1 A=2 (as measured by ASTM D 3493-76), cm.sup.3 /100 g; and PA1 Tint.sub.ASTM =tint strength (as measured by ASTM D 3265-76).
Tint residual shows good correlation with polydispersity estimates from electron microscopy and graduated ultrafiltration tests. A positive tint residual indicates a carbon black narrower in distribution and a negative tint residual indicates a carbon black broader in distribution than the average breadth of the blacks used in the development of Tint.sub.MODEL. It is important to note that tint residual is defined in terms of standard ASTM procedures rendering the test widely usable. Further, tint residual has been found to correlate well with such compounded properties of carbon black as heat build-up, resilience, and dynamic wire cord adhesion. Negative tint residuals correlate with blacks having broad distribution, low heat build-up, good resilience, and good dynamic wire cord adhesion. Nevertheless it is known that high positive tint residuals give better yields and theoretically better skid and traction. Accordingly, there is considerable demand for such high positive tint residual carbon blacks and methods and apparatus for producing such high positive tint residual carbon blacks are desirable. Such narrow distribution blacks are disclosed, for example, in U.S. Pat. No. 3,725,103.
By control of the conditions prevailing between time of introduction of the feedstock into the hot combustion gases and the manner and time of quenching to below carbon black forming temperatures, it is possible to control some of the factors or characteristics of the product carbon black. Because certain carbon blacks having certain characteristics, for example, iodine number determined according to ASTM D 1510-76, and tint strength number according to ASTM D 3265-76 are required for certain applications, it is desirable to have a method and apparatus for producing carbon black which provides means for changing or varying these characteristics to meet the required specifications. Since iodine number is inversely related and tint strength is directly related to "Cabot" tint factor, as disclosed, for example, in U.S. Pat. No. 3,725,103, it is particularly desirable to have a method whereby iodine number can be decreased and tint strength can be increased in order to facilitate production of such high Cabot tint factor carbon blacks as are disclosed in U.S. Pat. No. 3,725,103.
Accordingly, an object of the present invention is method and apparatus for producing carbon black having a narrow distribution. A further object is method and apparatus for varying the distribution of carbon blacks. A further object is method and apparatus for producing carbon blacks having a tint residual of +7 or greater. Yet another object is a method and apparatus for changing certain characteristics of carbon black such as iodine number and tint strength. Yet other objects and advantages will become apparent to one of ordinary skill in the art from the following disclosure and the claims.